In most modern telecommunication networks, a community of subscribers are connected to a central office switch through a "two-way" distribution network, which may include one or more transmission facilities, e.g., microwave, optical, electrical, etc., and which may utilize both analog and digital baseband transmission protocols. By way of example, subscribers associated with a respective central office switch may be connected to the switch via a series of remotely located network units, with each network unit serving subscribers in a defined local area. Between the central office switch and each network unit, telecommunication signals are commonly digitized and multiplexed for transport over relatively high bandwidth facilities for greater network efficiency. More recently, digital transmission of telecommunication signals may span all the way between a central office switch and a digital network termination point at a subscriber premise location.
In addition to basic analog or digital telephony, other types of services may also be provided over a traditional telecommunication network. For example, it is well known to use a digital/analog "modem" (modulator/demodulator) for transporting digital data over a telecommunication network via (relatively low frequency) analog modulation. Analog carrier also generally the transport mode of choice in many high bandwidth networks, such as video broadcast networks. For example, in a typical cable television ("CATV") broadcast network, a broadcast communication signal is transmitted "downstream" from a headend broadcast facility to a community of subscribers over a coaxial cable distribution network via RF carrier channels covering a wide (relatively high) frequency bandwidth--e.g., between 50 and 800 MHz.
In addition to the one-way, downstream distribution of the video broadcast signals, it is desirable in a CATV broadcast distribution network to be able to transmit and receive--i.e., in both the downstream and "upstream" directions,--information such as, e.g., network operations, administration and maintenance ("OAM") data, or "set-top" data. As used herein, "set-top" data refers generally to information transmitted to or from CATV control circuitry, which is traditionally located in a box-like unit placed on top of a subscriber's television set, hence the "set-top" designation, although more recently the control circuitry is incorporated within the television set itself. Downstream set-top data may include, for example, an instruction sent from the CATV service provider to activate or deactivate subscriber CATV service, or to authorize additional channel reception within the RF channel spectrum of the broadcast signal. Another example of downstream set-top data may include "polling" information to collect data on usage, e.g., for ratings or billing purposes. Upstream data sent from the subscriber set-top circuitry to the service provider, sometimes referred to as "set-top telemetry" data, may include a response to downstream polling, or a video-on-demand subscriber service request.
It is presently anticipated that several new services will arise requiring both point-to-point and point-to-multipoint transmission of independent communication signals, including a full range of digital baseband services. Thus, in addition to the desirability of combining traditional (synchronous) telecommunication and CATV services over a single distribution network, it is also desirable for such a network to fully support the two-way transport of multiple "broadband" data services, such as asynchronous transfer mode ("ATM") data packets, or "cells," Generally, ATM cells contain information relating to one or more communication signals, which are periodically assembled and transmitted from a sending node and received and disassembled at a receiving node. ATM transmission is particularly advantageous in that it enables the transport of multiple services over a single communication path, wherein bandwidth utilization may be optimized as a function of the statistical activity of each individual service--i.e., wherein the services are "bursty" in nature, so that bandwidth is efficiently shared. Other types of bursty data traffic include local area network ("LAN") traffic, which is traditionally limited to private, or closed-loop networks, but is more frequently being carried over shared public access (e.g., telecommunication) networks for greater efficiency in connecting multiple LAN locations. Further, with the explosion of recent interest in services associated with the "Internet", demand for low cost, high speed two-way digital data transport is at an all time high.
Given the wide variety of potential broadband communication services to be supported over a single, shared communication network, it is desirable to provide an efficient digital data transmission protocol for both downstream and upstream communication paths. Such a data transmission protocol should preferably be compatible with existing modem transmission standards, such as DAVIC and DVB standards, and should most fully utilize the available carrier channel bandwidth. The use of a coaxial distribution network to support a broadband data services network is desirable in that it is cost effective to use existing embedded plant, e.g., an existing CATV distribution network. Further, coaxial cable provides a relatively inexpensive mode for transporting high frequency communication signals, e.g., as opposed to twisted copper pair. However, because of the physical attributes of known coaxial distribution networks, transport capacity problems may arise due to transient noise, etc. Further, broadband data services being provided in addition to CATV broadcast and two-way telecommunication signals will have to contend for limited availability of bandwidth channels.
Thus, it is desirable to provide a data communication protocol that will dynamically and adaptively allocate limited network bandwidth to a number of communication nodes sharing a common communication medium, such as a relatively "noisy" coaxial distribution network, while simultaneously supporting multiple types of data traffic, including bursty data such as ATM or LAN traffic, as well as synchronous data such as continuous bit rate ("CBR") telephony traffic.